This invention relates to a voltage-controlled, variable speed inverter drive for an a-c motor, the construction of the inverter-motor system being substantially simplified compared to previously developed systems.
Many variable speed a-c motor drives comprise a forced-commutated thyristor inverter feeding an a-c motor. The thyristor switching devices (which usually take the form of silicon controlled rectifiers or SCR's) in the inverter are gated or turned on in a prescribed sequence in order to convert an applied d-c voltage, received from a d-c power supply, to a-c voltage for application to the a-c motor. Capacitors, additional switching devices or other commutating elements are required to turn off or commutate the conducting thyristors. Reactive diodes usually shunt the thyristors to provide a path for the circulation of reactive motor current. Of course, the inclusion of such forced commutation circuitry and reactive diodes adds considerable cost and complexity to the inverter.
It is known that the need for forced commutation is obviated by employing an a-c synchronous motor, which is constructed to present a leading power factor to the inverter drive, the alternating current in each of the motor stator windings thereby always leading the alternating motor voltage across the winding. Design techniques for constructing a synchronous motor to have a leading power factor are well understood in the art. Basically, it involves providing a motor back emf (electromotive force) that is greater than the applied inverter voltage. The back emf is induced in the stator windings by the rotating flux produced by the magnet (either a permanent magnet or an electromagnet) in the rotor. With a leading power factor the thyristors will be motor-commutated, meaning that when a thyristor is gated on it will cause the back emf to reverse bias and to turn off a previously conducting thyristor, the motor current thereby effectively transferring to the on-coming thyristor.
The previously developed synchronous motor-commutated thyristor inverter drives were fed by constant current sources. With such an arrangement, the current flowing through a conducting thyristor is maintained essentially constant during the entire on-time and until the thyristor is commutated off. During that on-time, a substantial amount of magnetic energy builds up and becomes locked in the equivalent inductance of the motor and this energy must be removed from the stator winding before the thyristor can be turned off. The removal of the energy from the stator winding is accomplished by two means in the conventional current source inverter. Some of the energy is transferred magnetically to a damper winding which, in effect, is a series of shorted turns on the rotor. At the time of commutation, currents are generated in these turns which sustain the flux that was previously supported by currents in the stator winding. Whatever energy that is stored in the stator flux fields which do not link the damper winding is transferred conductively to the back emf generator of the oncoming phase. Reference to FIG. 1 will help clarify the latter point.
In FIG. 1, a portion of a current source inverter is shown in which a constant current I is assumed to be flowing through a large inductor 10, SCR 11, motor inductance 12, equivalent motor voltage generators 13 and 14, motor inductance 15, and SCR 16. The motor inductances 12, 15, 17 are the equivalent commutating inductances which account for the flux fields that do not link the damper windings. At time, t.sub.o, shown on the accompanying graph, SCR 18 is assumed to be triggered. As is known in the art, if the motor power factor is leading, then the line-to-line motor voltage appearing across points 19 and 21 will have the polarity shown. This voltage is in the direction to cause current i.sub.22 to decay to zero and current i.sub.23 to build up as shown in the diagram. The large value of the d-c link inductance 10 maintains the input d-c current at the constant value of I during the commutation interval. As shown in the diagram, current i.sub.22 goes to zero and current i.sub.23 builds up to I. The line-to-line voltage appearing across points 19 and 21 reverse biases SCR 11 after current i.sub.22 has gone to zero. For successful commutation, the negative bias must last long enough to provide sufficient hold-off time for SCR 11.
The preponderance of the time to effect successful commutation is composed of the time t.sub.o and t.sub.1 during which time the energy and current transfer occurs from inductance 12 to inductance 17. This transfer time T will be given approximately by: ##EQU1## It is desirable to minimize the transfer time so that operation is efficient as possible and also to make possible operation at higher frequencies. Referring to equation 1, it is seen that transfer time T can be minimized by decreasing the value of the commutating inductance, decreasing the value of I or increasing the motor back emf.
In a conventinal current source inverter, I is set by the load torque required and in general cannot be decreased. The commutating inductance may be decreased by increasing the size of the damper winding. The motor voltage may be increased by increasing the back emf which implies larger magnets or electromagnets in the machine. Thus, decreasing the commutation interval is done by considerably compromising the cost and size of the synchronous machine. In addition, the conventional current source inverter also requires a very large inductor in the d-c link to maintain the current constant, often times being the same physical size as the motor.
These shortcomings of the prior thyristor inverter-motor systems have now been overcome by the present invention. A uniquely, and yet simply, constructed thyristor inverter-motor system is provided which is capable of driving high horsepower loads. A minimum number of circuit components are needed and no commutating elements, reactive diodes, large series inductor and damper winding are necessary. In short, the present invention provides a very efficient variable-frequency, voltage-controlled, inverter-motor system which is significantly simpler and less expensive in construction than any previous inverter-motor system.